In recent years, in radio communication, in particular, in mobile communication, various information such as images other than speech becomes targets of transmission. In accordance with diversification of transmission targets, it is required to realize a technique of transmitting information at high speed. OFDM (Orthogonal Frequency Division Multiplexing), which is one of the multicarrier transmission schemes, is attracting attention as a transmission scheme capable of responding to the above requirement.
OFDMA (Orthogonal Frequency Division Multiple Access), which is a multiple access scheme of OFDM, is a technique of dividing a frequency band available in a radio communication system into a plurality of bands and assigning each user (such as a mobile station apparatus) to one of the plurality of bands, and thereby achieving frequency multiplexing of a plurality of users.
Radio communication systems that apply OFDMA include an OFDMA-TDD (Orthogonal Frequency Division Multiple Access-Time Division Duplex) system. In bidirectional radio communication of a TDD scheme, the same frequency band is used for uplink and downlink, and therefore frequency use efficiency is high. Further, asymmetrical times are assigned to uplink and downlink to change the transmission rate, and therefore it is possible to flexibly respond to traffic changes.
In general, distances between a base station apparatus in a given cell and mobile station apparatuses that exists in the cell are different, and therefore the propagation delay times between the base station apparatus and the mobile station apparatuses are also different. Hereinafter, this difference of propagation delay times will be referred to as “propagation delay difference.” Normally, with a TDD scheme, mobile station apparatuses start transmission of uplink signals after downlink signals are received and a guard period (also referred to as “guard time”) has elapsed. At this time, due to a propagation delay difference, shift sometimes occurs in timings at which transmission signals of mobile station apparatuses arrive at a base station apparatus. As a result, intersymbol interference wherein the transmission signals of a plurality of mobile station apparatuses interfere with one another (hereafter “interference between users”) occurs.
As a method of preventing occurrence of intersymbol interference in radio communication of an OFDMA-TDD scheme, a method of controlling transmission timings of uplink signals is proposed (see, for example, Patent Document 1). In this transmission timing control method, uplink transmission timings are individually set per mobile station apparatus so that timings at which transmission signals of mobile station apparatuses arrive at a base station apparatus falls within a predetermined range. The set transmission timings are reported to mobile station apparatuses from a base station apparatus. Mobile station apparatuses transmit signals according to the reported timings.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI 11-113049